Caliber: 38.5 mm
Length: ? mm
Weight: ? kg
Maximum effective range: 200 meters
Tromboncino M28 rifle grenade launcher was an unique weapon, developed in 1927-28 for Italian military. It conceptually pre-dates modern underbarrel grenade launchers by several decades. However, this weapon had too many flaws and was in service with Italian army only between 1928 and 1934; after that, it was replaced with 45mm Brixia assault mortar, a stand-alone weapon of greater firepower and versatility. Major flaws of the Tromboncino M28 rifle grenade launcher were its significant bulk and weight, expensive construction, and inability to use launcher and its host rifle at the same time, due to single bolt being used alternatively in both weapons.
Tromboncino M28 rifle grenade launcher fired fin-stabilized fragmentation grenade, which was propelled using standard 6.5x52mm Carcano rifle cartridge. Complete grenade, known as S.R.2 bomb, weighted about 160 gram and had maximum effective range of 200 meters and limited blast radius.
The grenade launcher itself was a single shot, combination loaded weapon that was rigidly attached to the right side of the Carcano M91TS carbine (Moschetto Modelo 1891 per Truppe Speciali). Grenade is loaded from the front and held inside by a spring detent. Propelling cartridge is loaded from the rear, using the bolt removed from the host rifle to chamber it into the breech of grenade launcher. Trigger unit of the M28 launcher is linked to the trigger of the host carbine. Upon discharge, bullet of the propelling cartridge does not leave the firing chamber, being stopped almost immediately by a solid steel plug; powder gases are allowed to go forward through four ports around the plug to enter the grenade launcher bore and propel the grenade out and toward the target. Upon opening the bolt, empty case and fired bullet are ejected from the launcher. During this time, host carbine remains inoperable because its bolt is used in the launcher, and its trigger operates both sears (in carbine and in the launcher) at the same time. The grenade launcher sights are integrated into the carbine, with grenade rear sigh attached to the same base as carbine rear sight.